U.S. Pat No. 4,913,859 provides a general description of the manufacture of optical fiber as follows:
"In the manufacturing of optical fiber, a glass preform rod which generally is manufactured in a separate process is suspended vertically and moved into a furnace at a controlled rate. The preform softens in the furnace and optical fiber is drawn freely from the molten end of the preform rod by a capstan located at the base of a draw tower.
"Because the surface of the optical fiber is very susceptible to damage caused by abrasion, it becomes necessary to coat the optical fiber, after it is drawn, but before it comes into contact with any surface. Inasmuch as the application of the coating material must not damage the glass surface, the coating material is applied in a liquid state. Once applied, the coating material must become solidified rapidly before the optical fiber reaches a capstan. This may be accomplished by photocuring, for example.
"Those optical fiber performance properties which are selected most by the coating material are strength and transmission loss. Coating defects which may expose the optical fiber to subsequent damage arise primarily from improper application of the coating material. Defects such as large bubbles or voids, non-concentric coatings with unacceptably thin regions, or intermittent coatings must be avoided. When it is realized that the coating thickness may be as much as the radius of an optical fiber, it becomes apparent that non-concentricity can cause losses in splicing, for example.
"Transmission losses may occur in optical fibers because of a mechanism known as microbending. Optical fibers are readily bent when subjected to mechanical stresses, such as those encountered during placement in a cable or when the cabled fiber is exposed to varying temperature environments or mechanical handling. If the stresses placed on the fiber result in a random bending distortion of the fiber axis with periodic components in the millimeter range, Light propagating in the fiber core may escape therefrom. These losses, termed microbending losses, may be very large. Accordingly, the fiber must be isolated from stresses which cause microbending. The properties of the fiber coating play a major role in providing this isolation, with coating geometry, modulus and thermal expansion coefficient being the most important factors.
"Two types of coating materials are used to overcome this problem. Single coatings, employing a relatively high shear modulus, e.g. 10.sup.9 Pa, or an intermediate modulus, e.g. 10.sup.8 Pa, are used in applications requiring high fiber strengths or in cables which employ buffer tubes where fiber sensitivity to microbending is not a significant problem.
"Dual coated optical fibers increasingly are being used to obtain design flexibility and improved performance. A reduction in the modulus of the coating material reduces microbending sensitivity by relieving stress caused in the fiber. Typically, an inner or primary coating layer that comprises a relatively low modulus material, e.g. 10.sup.5 -10.sup.7 Pa, is applied to the optical fiber. The modulus of the primary coating should be effective in promoting long bending periods for the fiber which are outside the microbending range. Such a material reduces microbending losses associated with the cabling, installation or environmental changes during the service life of the optical fiber. In order to meet temperature conditions in expected areas of use, the low modulus coating material must be effective in the range of about -40.degree. to 77.degree. C. An outer or secondary coating layer comprising a relatively high modulus material is applied over the primary layer. The outer coating layer is usually of a higher modulus material to provide abrasion resistance and low friction for the fiber and the primary coating layer. The dual coating serves to cushion the optical fiber by way of the primary layer and to distribute the imposed forces by way of the secondary layers, so as to isolate the optical fiber from bending moments."
"After the coating material or materials have been applied to the moving optical fibers, the coating material or materials are cured, typically by exposure to ultraviolet radiation. In some coating systems, a primary coating material is applied and cured by subjecting it to ultraviolet energy prior to the application of the secondary coating material."
Other references pertaining to coatings for optical fibers include: U.S. Pat. Nos. 4,125,859; 4,474,830; 4,935,455; 4,946,874; 4,956,198; 4,973,611; 5,026,409; 5,139,872; 5,169,879; Martin, "Contribution of Dual UV Cured Coatings to Optical Fiber Strength and Durability," Proceedings, Radcure Europe '87, pp 4-15-4-24 (May 1987); Chawala, et al, "An Infrared Study of Water Absorbtion of UV Curable Optical Fiber Coatings," Radtech Report, 24-28January/February 1992; Smithgall, "A Dynamic Modal for Optical-Fiber Coating Application," J. Lightwave Technology, 8, 1584-1590 (October 1990); Simoff et al, "Thermo-Oxidative Aging of a Primary Lightguide Coating in Films and Dual-Coated Fibers," Polymer Engineering and Science, 29, 1177-1181 (Mid-September 1989); and Overton et al, "Time Temperature Dependence of Dual Coated Lightguide Pullout Measurements," Polymer Engineering and Science, 29, 1169-1171 (Mid-September 1989).
In U.S. Pat. No. 5,171,609 there is described a common problem encountered in curing of optical fiber coatings, that being the tendency of heat from high energy UV lamps typically employed to cure the coating material to evaporate some coating from the fiber and to deposit same on the wall of the transparent curing chamber, thereby darkening the chamber and reducing energy available to cure the coating. Various mechanical solutions to this problem have been proposed but all are complicated and cumbersome.